Nozzle and method for forming microdroplets

ABSTRACT

The invention relates to a nozzle for producing microdroplets of metal using gas flow, to a nozzle for producing microdroplets using electrodispersion, to a combination of a melt spinner for forming elongate metal fibers with a nozzle and to a method of forming microdroplets using at least one of a gas flow and electrodispersion.

The invention relates to a nozzle for producing metal droplets using gasflow and to a nozzle for producing metal droplets usingelectrodispersion. Furthermore, the invention relates to a combinationof a melt spinner for forming elongate metal fibers with a nozzleaccording to the invention and to a method of forming microdropletsusing at least one of gas flow and electrodispersion.

A known method to produce metal strands out of metal droplets is theprocess of melt spinning. Melt spinning is a technique used for rapidcooling of metal liquids. A thin stream of metal liquid is then drippedonto the circumferential surface of a fast rotating wheel where itundergoes rapid solidification. This technique is used to form elongatedstrands of materials such as metals or metallic glasses. The coolingrates achievable by melt-spinning are of the order of 10⁴-10⁷ Kelvin persecond (K/s). The process can continuously produce thin ribbons ofmaterial.

In this connection it should be noted that a strand can be understood asan element of which the length is at least twice its width, while thegeometry of its cross section may be round, oval, half-oval,rectangular, quadratic, triangular or such a related geometry.

A special role is assigned to metal strands and/or fibers with a lateraldimension in the micrometer range, i.e. 1 to 50 micrometers, and alength of several millimeters or centimeters. These materials, asindividual fibers, mesh of fibers or bunch of fibers, also incombination with other materials play a central role in a whole seriesof applications for the improvement of the most diverse properties.Examples of such applications are metallic wool and tissues,3-dimensional electrodes for batteries and accumulators, catalysis,conductive plastics for touch sensitive systems, such as, displays andartificial hands in the field of robots, anti-electrostatic textiles andplastics, mechanically reinforced textiles, plastics and cement forlightweight and heavy construction, filter materials for use inenvironments subjected to mechanical and/or chemical stress orcatalysis.

An important aspect for the improvement of metal strand based materialfunctions is a large surface area to weight ratio and the ability tomanufacture and process such strands in an industrially relevantprocess. This signifies: adjustable lengths, widths and cross sectiongeometries of metal strands, reproducibility and economic manufacturingmethods and low process costs with a high material yield per unit time.

Nowadays, the industrially relevant manufacture of functional materialsbased on metal strands is restricted to a strand width of about 50 μmand larger. These methods are based on drawing, template, rolling orextrusion processes. Fibers of stainless steel with a width of down to 8micrometers are manufactured by a complicated drawing process startingfrom a bundle of larger diameter fibers, which are drawn to smallerdiameters. For example, the fibers need to be coated with a layer ofcopper in order to allow gliding of the fibers along one another.However, these methods have disadvantages when being utilizedindustrially because they are restricted to a few materials only,require long process times and costly fabrication- and post-fabricationprocesses. The method is generally restricted to only a few kinds ofmetal.

Conventionally known apparatuses for producing metal strands viamelt-spinning processes usually let molten metal flow on a lateralsurface or a circumferential surface of a rotating wheel. This allowsthe metal melt to partly coat the rotating lateral or circumferentialsurface of the wheel with a certain thickness and to be “thrown off” thewheel as straight strand of defined thickness due to centrifugal forcesoriginating from the rotation of the wheel once the metal is solidified.

In conventional melt spinners a continuous flow of liquid metal isbrought in contact with the above mentioned rotating wheel to form themetal fibers. In order to be able to produce fibers in a micrometerscale it is well known that with finer continuous flows of molten metaleven smaller metal fibers can be formed on said rotating wheel. Hence,one approach to produce even smaller fibers is to reduce the size of anozzle opening, from which the molten metal is directed to the rotatingwheel, such that the flow of molten metal is reduced to a minimum.Nevertheless, it has shown that this approach has its limits, since thesize of the nozzle opening can only be reduced to a certain limitdepending on the surface tension of the molten metal, because at somepoint the molten metal would simply—because of said surface tension—notexit the nozzle anymore.

It is therefore an object of the invention to provide a nozzle and amethod for producing microdroplets of metal as well as a combination ofa melt spinner with a nozzle, with which the size of the produced fiberscan be decreased significantly in its width and length. This object issolved by the subject-matter of the independent claims.

By way of example a nozzle for producing microdroplets of metal, maycomprise a reservoir for molten metal, a nozzle opening for directingthe molten metal in a flow direction out of the reservoir and a channelconnecting the reservoir with the nozzle opening, wherein the nozzlefurther comprises an external force generating device configured toapply an external force on a molten metal flow flowing in said channelwith a force per unit area generated by the external force generatingdevice at the molten metal being larger than a surface tension of themolten metal.

A nozzle is thus provided into which a continuous or quasi continuousflow of molten metal is guided and in which nozzle the continuous orquasi continuous flow of molten metal is separated into individualbunches or droplets.

In other words, a nozzle is provided, which is configured to let moltenmetal flow out of a reservoir into one end of a channel. As the moltenmetal flow moves through the channel a force is applied on said moltenmetal flow in the channel or at the exit of the channel which so to saychops the molten metal flow into individual droplets, since the forceapplied on the molten metal flow is greater than a surface tension ofthe molten metal, if the force is not greater than the surface tension,the surface tension of the liquid molten metal would keep the flowcontinuous. By defining the speed at which the molten metal flow ischopped, the size of the droplets generated from the originallycontinuous molten metal flow can be controlled and thereby bepre-determined.

This means that the gist of the present invention is the application ofan external force, e.g. in the form of an externally applied gas flow orelectric field, to separate the molten metal flow into individualbunches of molten metal, whereas in prior art melt spinning applicationsthe flow molten metal flow is interrupted by controlling the speed ofthe moving surface moving relative to the molten metal flow.

As an alternative, depending on the temperature used gas may be replacedby a liquid stream which chops metal flow apart. The condition that thenapplies is that the liquid stream is not permitted to come into contactwith the rotating wheel in order to not interfere with the principle ofmetal fibre formation on the rotating wheel.

In particular, according to a first aspect of the invention a nozzle forproducing microdroplets of metal using gas flow is provided. The nozzlecomprises a reservoir for molten metal, a nozzle opening for directingthe molten metal in a flow direction out of the reservoir and a channelconnecting the reservoir with the nozzle opening. Furthermore, thenozzle comprises a gas flow generating device for generating anddirecting the gas flow to the molten metal through at least one supplyopening into the channel, wherein the supply opening is located at thenozzle opening or crossing the molten metal flow channel by a definedangle. A force per unit area, which is generated by the gas flow at themolten metal, is larger than the surface tension of the molten metal.

In other words, a nozzle is provided in which the external forcegenerating device is a gas supply by means of which the speed at whichthe molten metal flow is chopped is defined by the force of the gas flowon the molten metal flow.

In this connection it is noted that the reservoir can be a hollow space,which is configured to accommodate the molten metal. Hence, thereservoir can either be a tank filled with the molten metal or a kind ofa connecting piece, which can be attached to a separate tank and whichis configured to guide the molten metal from the tank into one end ofthe channel.

At a far end of the channel a nozzle opening is provided, through whichthe molten metal is directed in a flow direction. In order to be able toprovide microdroplets instead of a flow of molten metal, a gas flowgenerating device is provided, which is configured to generate a flow ofgas. Said flow of gas is then directed to the flow of molten metalinside the channel through a supply opening in the channel. The gassupply opening can be located in the near vicinity of the nozzleopening, i. e. for example not further away than 10 cm upstream from thenozzle opening, such that the gas flow is provided at the molten metalsomewhere in the channel, preferably at the point right before the meltexits the nozzle.

In this connection it is noted that in general the expression “nozzleopening” relates to lowermost point downstream the melt, where theformed droplet exits the nozzle.

A diameter of the gas supply opening can lie in the range of 0.001 to 5mm, preferably in the range of 0.005 to 0.015 mm. Such sizes of supplyopenings have been found beneficial in forming droplets of the desiredsize.

As already mentioned above, the force per unit area, which is applied atthe molten metal by the gas flow needs to exceed the surface tension ofthe molten metal such that microdroplets are formed from the metal flow,which then move on to exit the nozzle through the nozzle opening. Theinterfacial tensions of liquid metals can have values of up to and morethan 400 mN/m.

In this connection it is also noted that the gas flow can either beprovided as a continuous flow of gas, i.e. the same amount of gas issupplied for a period of five minutes or longer. Or the gas flow can beprovided as a “pulsed” gas flow, meaning that the gas flow pressure canbe modulated periodically, e.g. the gas flow is provided at a differentpressure for a millisecond. Naturally other cycle times of the pulsedgas can be provided. Furthermore, it is noted that the gas can be afluid with a comparatively high boiling temperature selected above 20°C., in particular above 100° C. This might be applicable for lowtemperature melting metals and metal alloys based on Gallium, Indium ortin. This means in other words that the gas flow can also be replaced bya high-boiling liquid.

According to a first embodiment of the invention the gas flow generatingdevice is configured to direct the gas flow perpendicular to or at anangle to the flow direction into the channel. Hence, the gas flow can bedirected to the molten metal from a side such that the gas flow can“cut” through the melt. In some cases it might help when the gas flow isnot provided at an exact perpendicular angle with respect to the flowdirection but rather at an angle smaller than 90° between the gas flowand the flow direction of the melt.

According to another embodiment the channel comprises two or more supplyopenings to receive the gas flow from more than one side around thecircumference of the nozzle. For some applications it can be helpful toprovide more than one gas flow in order to form micropdroplets out ofthe flow of metal melt.

In some cases, it might also be necessary to provide more than one gasflow in order to produce a plurality of microdroplets, which allcomprise roughly the same volume such that fibers, which can be producedout of the droplets, will all roughly comprise the same diameter.

In this connection it is noted that the gas in the gas flow can be air,Helium, N2, Ar2, CO2 or a combination from the above.

According to a second aspect of the invention a nozzle for producingmicrodroplets of metal is provided, in particular a nozzle as describedabove, using electrodispersion. The nozzle comprises a reservoir formolten metal, a nozzle opening for directing the molten metal in a flowdirection out of the reservoir and a channel connecting the reservoirwith the nozzle opening. Furthermore, the nozzle comprises a firstelectrode such as a metal piece and a device to apply an electric fieldbetween the first electrode and the molten metal with a force per unitarea generated by the electric field at the molten metal being largerthan a surface tension of the molten metal.

In other words, also according to the second aspect of the invention, anozzle is provided, which is configured to let molten metal flow out ofa reservoir into a channel and then to chop the molten metal flow in orexiting said channel through the application of an external force intodroplets of pre-determined size by varying the size of the force appliedat the flow of molten metal and then guiding the droplets out of thenozzle opening. In the present case the external force generating deviceis an electric field generator.

It should be noted in this connection that also other types of externalforce generating devices can be used provided that they can beconfigured to apply an external force on the molten metal flow which islarger than a surface tension of the molten metal flow so that acontinuous or quasi continuous molten metal flow can be separated intodroplets of pre-determinable size.

In this connection the reservoir can be a hollow space, which isconfigured to accommodate the molten metal. Hence, the reservoir caneither be a tank filled with the molten metal or some kind of aconnecting piece, which can be attached to a separate tank and which isconfigured to guide the molten metal from the tank into one end of thechannel.

At the far end of the channel a nozzle opening is provided, throughwhich the molten metal is directed in a flow direction. In order to beable to provide microdroplets instead of a flow of molten metal, a firstelectrode such as a metal piece and device to apply an electric fieldbetween the first electrode and the molten metal is provided. That is tosay, the device applies a voltage to the first electrode and the moltenmetal such that an electric field is generated between the firstelectrode and the molten metal. Said electric field induces a Coulombforce, which is able to break up the flow of molten metal intomicrodroplets.

The force per unit area, which is applied by said Coulomb force at themolten metal needs to exceed the surface tension of the molten metalsuch that microdroplets can be formed out of the metal flow. In thisconnection it is noted that interfacial tensions of liquid metals canvalue up and above 400 mN/m. The microdroplets can be formed directlyafter the melt exits the nozzle, i. e. right at the nozzle opening.

Furthermore, it is possible that the electric field is provided by thedevice such that the molten metal itself acts as a second electrode. Itis also possible that one electrode may be the molten metal itself whilethe second electrode the rotating wheel. In an alternative embodiment asecond—separate—electrode is provided at an opposite site of the moltenmetal such that the microdroplets are being formed when the molten metalflows through a space between said two electrodes. The second electrodecan also be a piece of metal or even a wheel of a melt spinner.

It is known that increasing the applied voltage or decreasing thedistance between two electrodes, i. e. for example the first electrodeand the molten metal or the second separate electrode, leads to a higherelectric field and thus also to a higher Coulomb force. Hence, there areseveral ways to increase the force per unit area high enough to overcomethe surface tension of the molten metal.

The first, and if existing the second electrode, can be, for example, apiece of metal or any other kind of suitable material composition suchas graphite powder as long as it is electrically conductive.

Depending on the exact composition of the molten metal it can in somecases also be favourable to provide two separate electrodes, whereas inother cases only one electrode with the molten metal itself acting asthe second electrode may be sufficient in order to continuously producethe microdroplets.

Generally, it is also possible to provide a nozzle, which comprises thegas generating device as well as the first electrode and a device togenerate an electric field between the first electrode and the moltenmetal flow such that a combination of both the gas flow and the electricfield can be used to form the microdroplets. Hence, it can also bepossible to produce plasma such that the plasma can “cut” droplets fromthe flow of molten metal exiting the nozzle. However, it is also notedthat usually some sort of gas is already present at the nozzle such thatplasma could already be produced by the use of a nozzle usingelectrodispersion. According to a first embodiment the first electrodecomprises an essentially cuboid shape with a length in the range of 1 to5 cm and a width in the range of 0.1 to 5 cm. In this connection it isnoted that the exact shape and size of the electrode may be veryvariable. Thus, different shapes and sizes can be chosen.

According to another embodiment the electric field generated between thefirst and the molten metal lies in the range of 1 V/cm to 1000 V/cm,preferably in the range of 10 V/cm to 800 V/cm, particularly in therange of 20 V/cm to 400 V/cm. The generated electric field only needs tobe high enough such that the force per unit area, which is applied bythe Coulomb force generated by the electric field at the molten metalexceeds the surface tension of the molten metal in order to producemicrodroplets out of the flow of molten metal.

It is further noted that the electric field can be generated by analternating current or a direct current. Hence, the force per unit areaapplied to the molten metal can either be applied continuously or in apulsed manner. However, applying alternating fields may limit the flowof current between the two electrodes.

According to another embodiment of the invention a cross-section of thechannel in the flow direction of the molten metal comprises arectangular or triangular shape. The precise shape of the channel may bechosen according to the composition of the molten metal and/or accordingto the type of nozzle, which is used, i.e. a nozzle using gas flow,electrodispersion, an external force generating device or maybe evencombinations of the foregoing.

Regardless of the choice of the cross section in the flow direction, thecross-section of the channel in a plane perpendicular to the flowdirection of the molten metal can be chosen to comprise a circular,rectangular, triangular, oval, polygonal or any other shape. Hence, thechannel can comprise a cylindrical, cuboid, pyramidal, conical or anyother shape. As already mentioned above, the tapered shapes can eitherbe tapered in the flow direction of the molten metal or also against theflow direction.

Also, the channel can comprise a length in the range of 0.1 to 100 mm,preferably 1 to 50 mm, in particular 5 to 20 mm. A limiting factor inthe choice of length of the channel may be the how fast the flow ofmolten metal cools down and thus solidifies. A solidification of themolten metal inside the channel has to be avoided. Hence, the length ofthe channel has to be chosen accordingly. Currently, for some metals, anapproximate length of about 10 mm has proven to be a preferable length.In this connection it is also noted that if a gas is used to produce themetal droplets, said gas may not be too cold such that the metalsolidifies before the droplets can be formed. Hence, it may be necessaryto heat the gas, which is used to “cut” the flow of molten metal.

According to still another embodiment the nozzle opening comprises acircular, rectangular, triangular, oval, polygonal or any other shapedcross-section. In this connection it is noted that the cross section ofthe nozzle opening can correspond to the cross section of the channel inthe plane perpendicular to the flow direction. Should this not be thecase, the channel further comprises a transition area, in which thecross section of the channel transitions to the cross section of thenozzle opening.

According to an embodiment a rectangular nozzle opening can comprise alength in the range of 0.5 to 10 cm, preferably 1 to 5 cm, and a widthin the range of 10 to 500 μm, preferably 20 to 200 μm, in particular 30to 100 μm.

According to another embodiment a circular nozzle opening can comprise adiameter of 10 to 500 μm, preferably 20 to 200 μm, in particular 30 to100 μm.

According to another embodiment of the invention the reservoir comprisesan inner shape, which is connected with the channel via a channelopening in the inner shape, wherein the inner shape of the reservoir isrounded or sloped the channel opening such that the molten metal isguided to the nozzle opening.

The formed microdroplets can comprise a diameter in the range of 0.010to 0.500 mm, preferably in the range of 0.050 to 0.150 mm.

Typical materials for the molten metal can be bronze, Au, Ag,cobalt-alloy, Fe-alloy, CuSi₁₋₁₅, AlSi1₋₁₅ or stainless steel.

In a third aspect of the invention a combination of a melt spinner forforming elongate metal fibers with a nozzle according to the inventionis provided. The melt spinner further comprises a rotatable wheel with acircumferential surface, at least one rotating planar surface andcollection means for collecting solidified fibers formed on one of thecircumferential surface and the rotating planar surface of the rotatablewheel from the molten metal and separated from the rotatable wheel byforces generated by the rotation of the rotatable wheel. Thus, themicrodroplets, formed in or at the nozzle, are directed from the nozzleopening to either one of the circumferential and the tangential surfaceof the rotating wheel. In both cases the drops will be—as soon as theytouch the respective surface of the wheel—elongated by the force of therotating wheel until it solidifies to a fiber. After solidification thefiber will be thrown off the wheel by a force generated by the wheel.Said force can for example be a circumferential force such that afterbeing thrown off the wheel, the collection means can catch thesolidified fibers.

In this connection it is noted that in the case that a nozzle usingelectrodispersion is provided, the rotating wheel itself, which isusually made out of metal, can act as the first electrode such that themolten metal acts as the second electrode.

State of the art melt spinners, which can be used with the nozzlesaccording to the invention are well known and are, for example,described in WO2017/042155 (which describes a so-called vertical meltspinner) and PCT/EP2020/063026 (which describes a so-called horizontalmelt spinner).

With said state of the art melt spinners typical distances between thenozzle and the rotating wheel lie in the range between 1 and 30 mm,whereas typical speeds for the rotating wheel lie in the range of 10 to100 m/s, preferably 20 to 75 m/s. This leads to contact times of thedroplets with the surface of the rotating wheel in the range of 1 to 10ms.

In a fourth aspect of the invention a method of forming microdropletsusing at least one of an external force field, a gas flow andelectrodispersion is provided. The method comprises the following stepsof providing a flow of molten metal at a nozzle opening and applying aforce per unit area at said nozzle opening on said flow of molten metal,with said force per unit area being larger than a surface tension ofsaid flow of molten metal. The nozzle can comprise the above mentionedfeatures of the invention such that the force per unit area, which isapplied at the nozzle opening originates from a gas flow and/orelectrodispersion.

The invention will now be described in further detail by way of exampleonly with reference to the accompanying drawings. In the drawings thereare shown:

FIGS. 1 a to 1 c : different examples of nozzles according to theinvention using gas flow;

FIG. 1 d : a further example of a nozzle according to the invention;

FIGS. 2 a to 2 d : different examples of nozzles according to theinvention using electrodispersion;

FIG. 3 : an example of a horizontal melt spinner;

FIG. 4 : an example of a vertical melt spinner;

FIGS. 5 a and 5 b : experimental results and pictures of producedmicrodroplets;

FIG. 6 scanning electron micrographs of a produced fibre;

FIG. 7 : a photograph of a cross section of a produced fiber;

FIGS. 8 a to 8 c : experimental results for distributions of fiberthicknesses and widths;

FIGS. 9 a to 9 c : experimental results for distributions of fiberthicknesses and widths;

FIG. 10 : a photograph of a produced bronze fiber;

FIG. 11 : a photograph of a plurality of produced bronze fibers;

FIGS. 12 a to 12 c : experimental results for distributions ofthicknesses and widths of produced fibers; and

FIG. 13 : experimental results for the variation of metal droplet volumeby controlling the gas pressure which chops the continuous flow ofmetals in metal droplets.

FIGS. 1 a to 1 c show different examples nozzles 10, each comprising areservoir 12 filled with molten metal 14, a channel 16 and a nozzleopening 18. It can be seen that the nozzle opening relates to the lastopening in a flow direction F of the molten metal 14, where the moltenmetal 14 actually leaves the nozzle 10.

The nozzles 10 shown in FIGS. 1 a to 1 c each furthermore comprise twosupply openings 20 for a gas flow, which is generated by a gas flowgenerating device (not shown). As can be seen the supply openings caneither supply said gas flow in a direction perpendicular to the flowdirection F of the molten metal 14 (see FIG. 1 a ) or at an anglesmaller than 90° between the flow direction F and a gas flow direction G(see FIGS. 1 b and 1 c ).

Regardless of the angle, the gas flow crosses the flow of molten metal14 right at the nozzle opening 18 or right before the molten metal 14exits the nozzle opening 18 such that a force per unit area generated bythe gas flow at the molten metal 14 exceeds the surface tension of themolten metal 14 to form microdroplets 22.

In this connection it is noted that the gas flow can either be acontinuous flow of gas or a pulsed flow of gas. The used gas can forexample be N₂, Ar₂ or another gas such as CO₂.

The supply openings 20 shown in FIGS. 1 a to 1 c each comprise adiameter in the range of 0.005 to 0.015 mm, whereas the channel 16comprises a diameter in the range of 0.050 to 0.250 mm. Even though theexpression “diameter” is used, it is clear that the cross section ofboth the supply openings 20 and the channel 16 do not necessarily haveto be circular but can also be polygonal, triangular, rectangular, ovalor any other shape.

The same applies also to the nozzle opening 18, which can have acircular cross section as well as a rectangular, triangular, oval orpolygonal one.

The reservoir 12 shown in FIGS. 1 a to 1 c is formed as a hollow space,which is configured to accommodate the metal melt. The hollow spacecomprises a channel opening 24 through which the metal melt 14 can flowinto the channel 16. An inner shape of the reservoir 12 is rounded atthe channel opening 24 such that the melt 14 can flow easily into thechannel 16.

The reservoir 12 can either be a tank, which holds a bigger volume ofmelt 14, or a connecting piece, which is configured to be attached to aseparate tank. Hence, the hollow space can either be big enough to holda bigger volume of melt 14 or just as big to act as a connecting piecebetween the channel 16 and a separate tank.

Another example of a nozzle 10 using a gas flow to produce themicrodroplets 22 is shown in FIG. 1 d . Also in this embodiment areservoir 12 for the molten metal is provided. The reservoir 12 furthercomprises a channel opening 24 through which a channel 16 is connectedto the reservoir. At the far end of the channel 16 the nozzle 10comprises a nozzle opening 18 through which the molten metal 14 canflow. Furthermore, one can see in FIG. 1 d that the channel 16 comprisestwo gas flow channels 17 with respective supply openings through whichone can direct a gas to the channel 16, which then chops the continuousflow of metal into a noncontinuous flow such that droplets exit theopening 18.

It should also be noted that the supply openings 20 could be arranged atthe nozzle opening 18 in order to separate the flow of molten metal 14at the nozzle opening 18.

In the design shown in FIG. 1 d , the two gas flow channels 17 first runin parallel to the channel 16 until they make a turn in the direction ofthe channel 16 such that they meet the channel at the respective supplyopenings 20 of the channel 16. They can either meet the channel 16 suchthat the gas, which flows through the gas flow channels 17 “cuts” themolten metal 14 perpendicular to the flow direction F or at anotherdefined angle.

In this connection it should be noted that the two gas flow channels 17could also be arranged in a different manner and extend e.g. obliquelywith respect to the channel 16 from their starting point.

Thus, it can be seen that the flow of gas can be provided at the nozzleat the reservoir 12 and in flow direction F. Such an embodiment can helpto reduce the space needed for the nozzle 10 since the gas flowgenerating device can be provided at the reservoir 12 and thus, does notneed any additional space next to the channel 16.

It is also possible that only one air flow channel 17 or more than twoair flow channels 17 are provided. Hence, it is noted that theembodiment of FIG. 1 d describes only an example and does not restrictthe invention in any way.

A typical diameter of said air flow channel is about 1 mm. Depending onthe type of metal used said diameter can also vary. The typical gaspressure, with which the gas flows through the air flow channel 17 andthrough the supply opening 18, lies in the range from 100 to 10000 mbar,preferably in the range of 800 to 1500 mbar. Said pressure can bedependent on the precise shape of the cross section of the air flowchannel as well as the channel for the molten metal.

FIGS. 2 a to 2 d show different examples of nozzles 10, which all usethe concept of electrodispersion to form microdroplets 22 of moltenmetal 14. The shown nozzles comprise generally the structure as thenozzles 10 of FIGS. 1 a to 1 c expect for the part with the supplyopening 20 since the nozzles 10 from FIGS. 2 a to 2 d do not need asupply opening of any kind.

However, said nozzles 10 also comprise a reservoir 12 filled with moltenmetal 14, a channel 16 and a nozzle opening 18. Furthermore, saidnozzles 10 comprise a first electrode 26, which can be designed inseveral different ways. As can be seen in FIGS. 2 c and 2 d said firstelectrode 26 is a separate piece of metal, which is placed near thenozzle opening 18. A typical value for the spacing between the nozzleopening 18 and the first electrode 26 lies in the range from 4 to 6 mm.FIG. 2 d additionally shows a second piece of metal, which is used as asecond electrode 30 such that the flow of molten metal 14 is guidedthrough a space between said two electrodes 26, 30 such that themicrodroplets 22 are formed therein.

As can be seen FIG. 2 c , on the other hand, it is not necessary toprovide a second separate electrode 30 since the molten metal 14 itselfcan act as the second electrode, meaning that the electric field isgenerated between the first electrode 26 and the molten metal 14.

In FIGS. 2 a and 2 b , on the other hand, the first electrode isrealized by a (metal) surface 28, onto which the formed microdroplets 22are directed. As will be described later in connection with FIGS. 4 and5 , said surface 28 can be a circumferential or tangential surface of arotating wheel of a so called melt spinner.

It should be noted that the channel 16 may comprises an approximatelength selected in the range of 5 to 30 mm, in particular in the rangeof 8 to 15 mm, whereas the diameter (or length) of the nozzle opening 18lies in that range of 0.005 to 0.100 mm.

FIG. 3 shows a typical horizontal melt spinner 32 for producing elongatemetal strands comprising a nozzle 10 with a nozzle opening 18, whichdeposits drops of molten metal 14 in a deposition direction D onto arotating planar surface 34 of a rotating wheel 36. In order to be ableto deposit molten metal, the nozzle 10 comprises a heating device 38,which heats the metal inside the nozzle 10 to a temperature where themetal is in its liquid state.

The nozzle opening 18 may be of any geometry, usually circular, oval,rectangular, quadratic or triangular. The opening width can lie in therange of 10 μm to 500 μm. The nozzle direction N may vary from 90° withrespect to the planar surface 34, i. e. it may be selected to lie in therange from 0° to 90°. Hence, the nozzle 10 could also be alignedparallel to the rotating planar surface 34 and still have a depositiondirection D which is perpendicular, or any other angle, to the planarsurface 34.

The diameter of the wheel 36 can range from centimeters to meters andthe wheel material maybe of any choice, which withstands the metal moltdeposition and fast rotation speed, in particular metal alloys such ascopper, copper alloys, brass, nickel, iron, iron oxide, stainless steelor carbon based material such as graphite or carbide, ceramic materials.It is also possible that the wheel 36 is a wheel of a base materialhaving a layer made of a metal or of a metal alloy of a ceramic materialor of graphite or a vapor deposited carbon, for example a copper wheel36 having a layer of graphite.

Because of the rotation of the wheel 36, the molten metal drops 22,which come into contact with the surface are entrained and therebyelongated by the wheel 36 to form elongate metal strands 40. Thesestrands 40 remain on the surface until they are cooled down enough tosolidify. For this purpose the rotating wheel 36 can be cooled by acooling device to for example room temperature or even below by coolingwith liquid nitrogen in order for the molten metal drops 22 to be ableto solidify to metal strands 40. If the wheel 36 was not cooled at allit would eventually heat up because of its contact with the (hot) moltenmetal 14 and hence prevent the molten metal 14 to cool down sufficientlyto solidify. Heating of the wheel can also affect its mechanicalstability. The cooling device C is shown inside the rotatable wheel 36,but it is noted that does not necessarily have to be located inside thewheel. There are sufficiently many methods known to cool such devices.

Once the metal fibers 40 are solidified the centrifugal forces which acton the metal fibers 40 due to the rotation of the wheel 36 will sufficein order to move the metal fibers 40 away from the planar surface. Asthe adhesion force between the solidified metal fibers 40 and the planarsurface is less than the force acting on the metal fiber 40 due to therotation of the planar surface. Thus, the solidified metal fibers 40 flyaway from the wheel 36 in a direction transverse to the circumference ofthe wheel 36.

For collection of the solidified fibers 40 collection means 42 areprovided, which basically catch the fibers 40 flying away from therotating wheel 36.

A typical vertical melt spinner is shown in FIG. 4 . Since the verticalmelt spinner comprises several components, which are identical to theones from the horizontal melt spinner, only the differences betweenthese two will now be described.

The difference lies in the alignment of the rotating wheel 36 and hencethe corresponding surface onto which the microdroplets 22 are guided.While the rotating wheel 36 of the horizontal melt spinner of FIG. 3 isaligned such that the microdroplets are being guided on one of itsplanar lateral surfaces 34, in the vertical melt spinner the rotatingwheel 36 is aligned such that the micropdroplets 22 are guided onto thecircumferential surface 35 of the wheel. Hence, a rotation axis A of therotating wheel 36 is aligned perpendicular to the flow direction F ofthe molten metal 14, whereas the rotation axis A of the horizontal meltspinner is aligned parallel to the flow direction F of the melt spinner.

Regardless of the alignment of the rotating wheel 36, the microdroplets22 are elongated by the rotating wheel 36 just as described before inconnection with FIG. 4 .

FIGS. 5 a to 12 c show different photographs and experimental results ofthe produced microdroplets 22 as well as the therewith produced fibers40.

FIG. 5 a shows a photograph of microdroplets 22, which are composed ofbronze, whereas FIG. 5 b shows experimental results for a sizedistribution of the diameter of microdroplets 22, which are composed ofa cobalt-alloy. The diameter for both materials was constant throughoutthe experiment and in the range of 0.060 to 0.250 mm. It has furthershown that the ejection of the droplets 22 can also be held constant inthe range of 1 to 10 ms depending on the precise experimental settings.An increase of pressure, for example, has shown to have a minorinfluence on the microdroplet diameter, but a notable influence on thetime laps between the ejection of two microdroplets.

The solidified fiber, which results from guiding the cobalt-alloymicrodroplet 22 on a rotating wheel 36 of a horizontal or vertical meltspinner is shown in FIG. 6 . It is observed that a small droplet remainsat the very end of the produced fiber with a width of approximately 60μm. Said remaining droplet is shown in detail in the three bottomphotographs. The width of the produced fiber is approximately 12 μm.

A cross section of a produced bronze fiber is shown in FIG. 7 . It canbe seen that a typical cross section is asymmetric and comprises astraight part 44, which is contact with the wheel surface as well as acurved part 46 at the opposite side of the fiber 40. The parts with thehighest curvature (left and right on the picture) result of poor wettingof the rotating wheel 36 by the melt 14. The maximal height of the fiberin this photograph is about 6 μm.

Size distributions of the fiber thicknesses and widths are shown inFIGS. 8 a to 9 c . The distributions are quite narrow and usually eitherGaussian or log-normal distributions. Experiments have shown thatparameters such as the wheel speed, roughness of the wheel surface,temperature of the melt and so on, can influence the landing of themicrodroplet 22 on the wheel surface and thus the formation of thefiber. For a cobalt-alloy, the comparison of the size distributionsindicates that the wheel surface speed influences the fiber geometrysignificantly. If the droplet diameter is kept constant as well as otherexperimental parameters, the increase of the wheel surface speed from 25m/s to 50 m/s results in a decrease of thickness of 50% and decrease ofwidth of 30% (comparison of FIGS. 8 a to 8 c with 9 a to 9 c and FIG. 12).

If the microdroplet 22 diameter is reduced to 60 μm (see FIG. 5 a ), thewidth of the fabricated fiber at standard experimental conditions issignificantly below 10 μm (see FIG. 10 ). A picture of a large amount offibers produced with the said experimental settings is shown in FIG. 11.

The dropping process and its stability have shown to depend on thephysical properties of the materials in contact at the microscopicscale, i.e. viscosity and surface tension of the melt, wetting of thenozzle surfaces by the melt (often sharply depending on the temperature)and the mechanical properties of the nozzle surfaces.

At the minimum required pressure difference and slightly above, threequalitative experimental observations are of crucial importance tosupport the dropping of the microdroplets: Firstly, the melt should notwet the nozzle (i. e. wetting angle>>90° but between contact angles from0° to 90°);

Secondly, a reduced roughness of the surface of the rotating wheel is ofadvantage to improve the process stability. If the roughness of thenozzle surface is in the range of 0.05 mm, it introduces a heterogeneousflow of melt. Hence, polishing the surface with a polishing paper, suchas sandpaper with a grit size of down to a grain size of 0.003 mm hasshown to be beneficial. In this connection sand paper with a grit sizeselected in the range of 20 to 500 can be selected preferably with agrit size of around 200 to 350.

As a third point experiments have shown that the borders of the nozzleopening should be as sharp as possible, i. e. rounded borders favordroplets of lager diameter. Hence, the sharper the borders, the smallerthe microdroplets can get.

Finally, FIG. 13 shows how the microdroplet volume varies if the gaspressure of the gas flow, which chops the continuous flow of metal indroplets is controlled to different values. One can clearly see that acontrol of the gas pressure is crucial in order to produce microdropletswith a volume down to several nanoliters.

Generally speaking, the volume of the microdroplets lies in the range of0.1 to 20 nanoliters, in particular in the range of 2 to 9 nanoliters.In this connection a nozzle pressure, i. e. the pressure, with which thegas, e.g. Argon, is supplied in the channel to the molten metal, can beselected in the range of 900 mbar to 2000 mbar, in particular in therange of 1000 to 1500 mbar for a crucible pressure of 880 to 1050 mbar,in particular in the range of 900 to 1000 mbar, with the specific valuesbeing indicated in the drawing of FIG. 13 . The term “crucible pressure”relates to the pressure exerted on the molten metal in the channel ofthe molten metal. In this connection it is noted that the nozzlepressure should not be much greater than the crucible pressure, asotherwise the molten metal can flow back into the gas passage for thegas flow.

Control of the Liquid Metal Droplet Volume by Electrodispersion

As already mentioned above, the interfacial tension of liquid metals isvery large, i.e. >400 mN/m. Therefore, liquid metals tend to formdroplets in gas atmosphere or liquids to minimize their surface energy.

Electrodispersion techniques utilize a high electrical voltage toovercome the surface tension of a liquid meniscus at a orifice, allowingthe breaking of the liquid into either monodisperse or polydisperse finedroplets.

Breakup of emerging liquid metal droplet occurs when the disruptiveforces, i.e. the Coulomb force induced by the high voltage, overcome theinterfacial tension that resists deformation of the droplet.

The liquid metal phase acts as the second electrode while for example apiece of solidified metal surface is the first electrode. In between thetwo electrodes an electric field is generated by a (not shown) device.Said electric field then generates a Coulomb force, which can overcomethe interfacial tension that resists deformation of the droplet.Increasing the voltage or decreasing the distance between the twoelectrodes increases the electric field and thus also the Coulomb force,which is acting at the liquid interface.

Electrodispersion in Combination with Melt Spinning

The control of a liquid metal droplet volume, which exits the nozzle ofa crucible by electric fields can directly be applied to control thedimension of ultrafine metal fibers. Therefore, in some embodiments ofthe invention the liquid metal is contacted by one electrode while thesecond electrode is the rotating metal wheel or an electrode, which isbrought close to the exit of the nozzle. The thereby formed dropletswith controlled volume are brought in contact with the rotating wheel,as described above, and an ultrafine metal fiber is pulled out of thedroplet, which is in contact with the fast rotating wheel.

Control of Liquid Metal Droplet Volume by Gas Flow

A strong gas flow, which is brought close to the exit of a nozzle andcrosses the linear flow of liquid metal from the exit, can overcome thesurface tension of a fluid meniscus at an orifice, allowing the breakingof the liquid into either monodisperse or polydisperse fine droplets.The possible set up for a nozzle as used in this setup has beenexplained above in connection with FIGS. 1 to 4 .

Generally, it is imaginable that both described methods, i.e.electrodispersion and gas flow, are realized in one single nozzle. Withsuch a nozzle, the used method could for example be chosen according tothe composition of the used molten metal. In some cases it can also beuseful to apply both methods at the same time. Hence, the option tochoose is can be given.

1.-18. (canceled)
 19. A nozzle for producing microdroplets of metal, thenozzle comprising a reservoir for molten metal, a nozzle opening fordirecting the molten metal in a flow direction out of the reservoir anda channel connecting the reservoir with the nozzle opening, wherein thenozzle further comprises an external force generating device configuredto apply an external force on a molten metal flow flowing in saidchannel with a force per unit area generated by the external forcegenerating device at the molten metal being larger than a surfacetension of the molten metal.
 20. A nozzle for producing microdroplets ofmetal using gas flow, the nozzle comprising a reservoir for moltenmetal, a nozzle opening for directing the molten metal in a flowdirection out of the reservoir and a channel connecting the reservoirwith the nozzle opening, wherein the nozzle further comprises a gas flowgenerating device for generating and directing the gas flow to themolten metal through at least one supply opening into the channel,wherein the gas supply opening is located at the nozzle opening, whereina force per unit area generated by the gas flow at the molten metal islarger than the surface tension of the molten metal.
 21. The nozzleaccording to claim 20, wherein the gas flow generating device isconfigured to direct the gas flow perpendicular to or at an angle to theflow direction of the channel.
 22. The nozzle according to claim 20,wherein the channel comprises two or more supply openings to receive thegas flow from more than one side around the circumference of the nozzle.23. The nozzle according to claim 20, wherein the gas in the gas flow isair, argon, Helium, N2, Ar2, CO2 or combinations of the foregoing.
 24. Anozzle for producing microdroplets of metal using electrodispersion, thenozzle comprising a reservoir for molten metal, a nozzle opening fordirecting the molten metal in a flow direction out of the reservoir anda channel connecting the reservoir with the nozzle opening, wherein thenozzle further comprises a first electrode such as a metal piece and adevice to apply an electric field between the first electrode and themolten metal with a force per unit area generated by the electric fieldat the molten metal being larger than a surface tension of the moltenmetal.
 25. The nozzle according to claim 24, wherein the first electrodecomprises an essentially cuboid shape.
 26. The nozzle according to claim24, wherein the electric field generated between the first electrode andthe molten metal has a field strength which lies in the range of 1 V/cmto 1000 V/cm.
 27. The nozzle according to claim 24, wherein the electricfield is generated by one of an alternating current and a directcurrent.
 28. The nozzle according to claim 20, wherein a cross-sectionof the channel in the flow direction of the molten metal comprises oneof a square shape, a rectangular shape, a round shape, an oval shape, apolygonal shape and a triangular shape.
 29. The nozzle according toclaim 20, wherein the cross-section of the channel in a planeperpendicular to the flow direction of the molten metal comprises acircular, rectangular, triangular, oval, or polygonal shape.
 30. Thenozzle according to claim 20, wherein the channel comprises a length inthe range of 0.1 to 100 mm,
 31. The nozzle according to claim 20,wherein the nozzle opening comprises a circular, oval, square,rectangular, triangular, polygonal or any other shaped cross-section.32. The nozzle according to claim 31, wherein a rectangular nozzleopening comprises a length in the range of 0.5 to 10 cm; or wherein acircular nozzle opening comprises a diameter of 10 to 500 μm, preferably20 to 200 μm, in particular 30 to 100 μm.
 33. The nozzle according toclaim 20, wherein the reservoir comprises an inner shape, which isconnected with the channel via a channel opening in the inner shape,wherein the inner shape of the reservoir is rounded or sloped at thechannel opening such that the molten metal is guided to the nozzleopening.
 34. The nozzle according to claim 20, wherein the formedmicrodroplets comprise a size in the range of 0.010 to 500 mm.
 35. Acombination of a melt spinner for forming elongate metal fibers with anozzle according to claim 20, wherein the melt spinner further comprisesa rotatable wheel with a circumferential surface, at least one rotatingplanar surface and collection means for collecting solidified fibersformed on one of the circumferential surface and the rotating planarsurface of the rotatable wheel from the molten metal and separated fromthe rotatable wheel by forces generated by the rotation of the rotatablewheel.
 36. A method of forming microdroplets using at least one of anexternal force field, a gas flow and electrodispersion, wherein themethod comprises the following steps: providing a flow of molten metalat a nozzle opening; and applying a force per unit area at said nozzleopening by means of one of the on said flow of molten metal, with saidforce per unit area being larger than a surface tension of said flow ofmolten metal.
 37. The nozzle according to claim 19, wherein thecross-section of the channel in a plane perpendicular to the flowdirection of the molten metal comprises a circular, rectangular,triangular, oval, or polygonal shape.
 38. The nozzle according to claim19, wherein the nozzle opening comprises a circular, oval, square,rectangular, triangular, polygonal or any other shaped cross-section.